The present invention relates to a system for managing the heat from a heat source like an electronic component. More particularly, the present invention relates to a system effective for dissipating the heat generated by an electronic component.
With the development of more and more sophisticated electronic components, including those capable of increasing processing speeds and higher frequencies, having smaller size and more complicated power requirements, and exhibiting other technological advances, such as microprocessors and integrated circuits in electronic and electrical components and systems as well as in other devices such as high power optical devices, relatively extreme temperatures can be generated. However, microprocessors, integrated circuits and other sophisticated electronic components typically operate efficiently only under a certain range of threshold temperatures. The excessive heat generated during operation of these components can not only harm their own performance, but can also degrade the performance and reliability of the overall system and can even cause system failure. The increasingly wide range of environmental conditions, including temperature extremes, in which electronic systems are expected to operate, exacerbates these negative effects.
With the increased need for heat dissipation from microelectronic devices caused by these conditions, thermal management becomes an increasingly important element of the design of electronic products. As noted, both performance reliability and life expectancy of electronic equipment are inversely related to the component temperature of the equipment.
For instance, a reduction in the operating temperature of a device such as a typical silicon semiconductor can correspond to an exponential increase in the reliability and life expectancy of the device. Therefore, to maximize the life-span and reliability of a component, controlling the device operating temperature within the limits set by the designers is of paramount importance.
Heat sinks are components that facilitate heat dissipation from the surface of a heat source, such as a heat-generating electronic component, to a cooler environment, usually air. In many typical situations, heat transfer between the solid surface of the component and the air is the least efficient within the system, and the solid-air interface thus represents the greatest barrier for heat dissipation. A heat sink seeks to increase the heat transfer efficiency between the components and the ambient air primarily by increasing the surface area that is in direct contact with the air. This allows more heat to be dissipated and thus lowers the device operating temperature. The primary purpose of a heat sink is to help maintain the device temperature below the maximum allowable temperature specified by its designer/manufacturer.
Typically, heat sinks are formed of a metal, especially copper or aluminum, due to the ability of copper to readily absorb heat and transfer it about its entire structure. In many applications, copper heat sinks are formed with fins or other structures to increase the surface area of the heat sink, with air being forced across or through the copper fins (such as by a fan) to effect heat dissipation from the electronic component, through the copper heat sink and then to the air.
Limitations exist, however, with the use of copper heat sinks. One limitation relates to copper""s relative isotropyxe2x80x94that is, the tendency of a copper structure to distribute heat relatively evenly about the structure. The isotropy of copper means that heat transmitted to a copper heat sink become distributed about the structure rather than being directed to the fins where most efficient transfer to the air occurs. This can reduce the efficiency of heat dissipation using a copper heat sink. In addition, the use of copper or aluminum heat sinks can present a problem because of the weight of the metal, particularly when the heating area is significantly smaller than that of the heat sink. For instance, pure copper weighs 8.96 grams per cubic centimeter (g/cc) and pure aluminum weighs 2.70 g/cc (compare with pure graphite, which weighs between about 1.4 and 1.8 g/cc). In many applications, several heat sinks need to be arrayed on, e.g., a circuit board to dissipate heat from a variety of components on the board. If copper heat sinks are employed, the sheer weight of copper on the board can increase the chances of the board cracking or of other equally undesirable effects, and increases the weight of the component itself. In addition, since copper is a metal and thus has surface irregularities and deformations common to metals, and it is likely that the surface of the electronic component to which a copper heat sink is being joined is also metal or another relatively rigid material such as aluminum oxide or a ceramic material, making a complete connection between a copper heat sink and the component, so as to maximize heat transfer from the component to the copper heat sink, can be difficult without a relatively high pressure mount, which is undesirable since damage to the electronic component could result.
What is desired, therefore, is a thermal management system effective for dissipating heat from a heat source such as an electronic component. The thermal management system should advantageously be relatively anisotropic as compared to copper, exhibit a relatively high ratio of thermal conductivity to weight, and be capable of conformable mating with the surface of the heat source.
It is an object of the present invention to provide a thermal management system for a heat source, the thermal management system being effective to increase the useful area of the surface of the heat source from which heat is to be dissipated.
Another object of the present invention is to provide a thermal management system exhibiting a relatively high degree of anisotropy.
Still another object of the present invention is to provide a thermal management system capable of forming a desirably complete thermal connection with the surface of the heat source without the need for a high pressure mount.
Yet another object of the present invention is to provide a thermal management system having a relatively high ratio of thermal conductivity to weight.
Still another object of the present invention is to provide a thermal management system that can be fabricated so as to locate the heat dissipation surfaces thereof so as to maximize the dissipation of heat from the heat source.
These objects and others that will become apparent to the artisan upon review of the following description can be accomplished by providing a heat source, such as an electronic component, the heat source having an external surface. A thermal interface mounted to the external surface of the heat source is also provided, where the thermal interface comprises an anisotropic flexible graphite sheet having a planar area greater than the area of the external surface of the heat source. In another embodiment of the invention, the thermal management system includes a heat sink which comprises a graphite article shaped so as to provide a heat collection surface and at least one heat dissipation surface, wherein arranging the heat collection surface of the graphite article in operative connection with a heat source causes dissipation of heat from the heat source through the at least one heat dissipation surface of the graphite article. The graphite article useful as the heat sink of this embodiment of the invention comprises compressed particles of exfoliated graphite, such as anisotropic flexible sheets of compressed particles of exfoliated graphite laminated into a unitary article or particles of exfoliated graphite compressed into a desired shape. In addition, the graphite article can be formed of high density graphite fabricated from finely divided carbonaceous particles.
Graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size, the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. Graphites possess anisotropic structures and thus exhibit or possess many properties such as thermal conductivity that are highly directional. Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two sets of axes or directions are usually noted, to wit, the xe2x80x9ccxe2x80x9d axis or direction and the xe2x80x9caxe2x80x9d axes or directions. For simplicity, the xe2x80x9ccxe2x80x9d axis or direction may be considered as the direction perpendicular to the carbon layers. The xe2x80x9caxe2x80x9d axes or directions may be considered as the directions parallel to the carbon layers (parallel to the planar direction of the crystal structure of the graphite) or the directions perpendicular to the xe2x80x9ccxe2x80x9d direction.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Graphites can be treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the xe2x80x9ccxe2x80x9d direction and thus form an expanded graphite structure (also referred to as exfoliated or intumesced graphite) in which the laminar character of the carbon layers is substantially retained.
Graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or xe2x80x9ccxe2x80x9d direction dimension which is up to about 80 or more times the original xe2x80x9ccxe2x80x9d direction dimension can be formed without the use of a binder into cohesive or integrated articles and flexible graphite sheets of expanded graphite, e.g. webs, papers, strips, tapes, or the like. The formation of graphite particles which have been expanded to have a final thickness or xe2x80x9ccxe2x80x9d dimension which is up to about 80 or more times the original xe2x80x9ccxe2x80x9d direction dimension into integrated articles and flexible sheets by compression, without the use of any binding material, is believed to be possible due to the excellent mechanical interlocking, or cohesion, which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the graphite material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal conductivity, comparable to the graphite starting material due to orientation of the expanded graphite particles substantially parallel to the opposed faces of the sheet resulting from high compression, e.g. roll pressing. Sheet material thus produced has excellent flexibility, good strength and a very high degree of orientation.
Generally, the process of producing flexible, binderless anisotropic graphite sheet material, e.g. web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a xe2x80x9ccxe2x80x9d direction dimension which is up to about 80 or more times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles that generally are worm-like or vermiform in appearance, once compressed, will maintain the compression set and alignment with the opposed major surfaces of the sheet. The density and thickness of the sheet material can be varied by controlling the degree of compression. The density of the sheet material can be within the range of from about 0.05 grams per cubic centimeter to about 1.8 grams per cubic centimeter. The flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon roll pressing of the sheet material to increased density. In roll pressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the xe2x80x9ccxe2x80x9d direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprising the xe2x80x9caxe2x80x9d directions and the thermal properties of the sheet are very different, by more than an order of magnitude, for the xe2x80x9ccxe2x80x9d and xe2x80x9caxe2x80x9d directions (i.e., about 7 watts per meter-xc2x0 C. (w/mxc2x0 C.) vs. about 150-200 w/mxc2x0 C).
With respect to thermal properties, the thermal conductivity of a flexible graphite sheet in a direction parallel to the upper and lower surfaces of the flexible graphite sheet is relatively high, while it is relatively very low in the xe2x80x9ccxe2x80x9d direction transverse to the upper and lower surfaces, by a ratio that can approach 20 to one or higher.
Similarly, a compressed expanded graphite article can be produced by compressing or compacting under a predetermined load (and again without the necessity of a binder), expanded graphite particles. The particles can be compressed into a mold under pressures of at least about 100 pounds per square inch (psi), and will retain their final shape and exhibit significant and measurable anisotropy.